Experimental investigation on drying performance of an existed enclosed fixed frequency air source heat pump drying system

Highlights: • System drying performance influenced by air flow ratio changing was investigated. • Drying performances of adjusting air temperature and flow ratio was compared. • Drying time decreased 42 min, or 15.0%, by the strategy of hot air bypassed. • Energy consumption of compressor was shorted from 4.27 kWh to 3.63 kWh. • Contributions of this study can guide low temperature material drying process. - Abstract: As the quick development of industry drying technology, different heat pump drying systems were proposed in recent decades. Enclosed heat pump drying system was considered as the most widely used system. For an existed enclosed fixed frequency heat pump drying system, drying time could be decreased by increasing air temperature at inlet of drying chamber. However, as a fundamental problem, system drying performance influenced by air flow ratio was not tested and reported in open literatures. Therefore, basing on adding an air bypass duct, drying performance of an enclosed system was experimental investigated in this study, with 15 mm thickness fresh carrot chips used. Furthermore, qualitatively and quantitatively comparisons and discussions on experimental results were conducted. A whole drying process were firstly divided into three stages by different water content ratios, preheating stage at 98–100%, fast drying stage at 20–98%, and later drying stage at 0–20%, respectively. For the inlet air temperature of drying chamber is fixed at 40 °C, material drying time for water content ratio reaching 20% could be effectively decreased as much as 42 min, or 15.0%, by the strategy of hot air bypassed. After the drying time shortened, the calculated energy consumption for compressor was also decreased from 4.27 kWh for AFR at 1.0 to 3.63 kWh for AFR at 0.6. Contributions of this study can guide low temperature material drying process. Clearly, system control optimization and energy saving were both expected.